Use of the Ethernet has been spreading with explosion of the Internet traffic. As to the Ethernet which is internationally standardized in the IEEE, the communication speed has been increasing from 10M initially to 100M, 1G and to 10G. The transmission distance of the Ethernet is 40 km at most. Thus, long distance transmission is not available by itself. OTN (Optical Transport Network) defined by ITU-T is an international standard technique for performing wide area transmission of various client signals including the Ethernet with high reliability (refer to the non-patent document 1, for example). By accommodating the Ethernet to the OTN, it becomes possible to perform long distance transmission exceeding 40 km.
FIG. 6 is a block diagram of a conventional technique for multiplexing a plurality of client signals to the OTN. In FIG. 6, each of the shaded areas indicates an independent clock domain. The client signal accommodating and multiplexing apparatus shown in the figure includes an OTN transmission side (upper part of the figure) and an OTN receiving side (lower part of the figure). The OTN transmission side includes a plurality of client signal receiving parts 11, accommodation parts 12 each being connected to the client receiving part 11, a multiplexing part 13, a framer part 14, an OTN signal transmission part 15. The OTN receiving side includes an OTN signal receiving part 16, a framer part 17, a separation part 18, a plurality of restoration parts 20, a plurality of PLL (Phase Locked Loop) parts 19 each being connected to the restoration part 20, and a plurality of client signal transmission parts 21 each being connected to the restoration part 20.
O/E conversion is performed on the client signal such as the Ethernet signal by the client signal receiving part 11. After that, the client signal is mapped to an ODU suitable for respective client signals by the accommodation part 12, and further the ODU is mapped to the ODTU. BMP (Bit-synchronous Mapping Procedure), AMP (Asynchronous Mapping Procedure), GMP (Generic Mapping Procedure) or the like is used for the mapping of the client signal to the ODU. Also, AMP, GMP or the like is used for mapping of the ODU to the ODTU. The multiplexing part 13 multiplexes a plurality of ODTUs into an upper ODU. Then, the framer part 14 performs overhead processing and adds error correction code so as to generate an OTU signal. Finally, the OTN signal transmission part 15 generates an optical transmission signal and transmits it. In the receiving side of the OTN signal, an OTU signal is obtained by performing O/E conversion on a signal received by the OTN signal receiving part 16. The framer part 17 performs overhead processing and decoding of the error correction code for the OTU signal so as to send an ODU signal to the separation part 18. Next, the separation part 18 separates the ODU signal into a plurality of ODTU signals. The separated ODTU signal is de-mapped by the restoration part 20 to generate an ODU signal, and the ODU signal is de-mapped to the client signal. The client signal transmission part 21 performs E/O conversion on the client signal and transmits it.
In recent years, along with the popularization of the Ethernet, the OTN standard has been largely expanded to weigh heavily on the Ethernet transfer (refer to non-patent document 2, for example). More particularly, ODU0 for accommodating GbE and ODU2e for accommodating 10 GbE are defined, as new ODUs (Optical Channel Data Unit). By multiplexing these ODUs to an upper ODU (ODU3, ODU4, for example), it becomes possible to perform economic wide area transfer of the Ethernet.